NATO SfP Scintillators NEW SCINTILLATOR MATERIALS FOR SCIENTIFIC, MEDICAL AND INDUSTRIAL APPLICATIONS

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1 NEW SCINTILLATOR MATERIALS FOR SCIENTIFIC, MEDICAL AND INDUSTRIAL APPLICATIONS NATO SfP Project no Scintillators. Performed in April 2 October 23 Final Report Project Co-Directors: 1. Dr. Gian Paolo Pazzi (NPD) Institute of Applied Physics "N. Carrara" IFAC-CNR (formerly IROE-CNR) Florence Italy 2. Dr. Anna Vedda Dept. of Material Science - University of Milano-Bicocca Milan Italy 3. Dr. Martin Nikl (PPD) Institute of Physics AS CR Prague Czech Republic 4. Prof. Svetlana Zazubovich Institute of Physics - University of Tartu Tartu Estonia Industrial partner - end user: 5. CRYTUR Ltd. Ing. Karel Blazek Director Turnov Czech Republic 1

2 Table of contents page List of abbreviations 3 1. Introduction 4 2. Scope and objectives of the project 5 3. Realization of the project 6 4. Scientific results Ce-doped aluminium perovskites Ce-doped aluminium garnets Doped PbWO RE-doped sodium-gadolinium phosphate glasses Impact of the project on the present and future research activities in the participating teams Meeting the criteria for success and Implementation of the results Conclusion 29 Annex 1. List of collaborators 3 Annex 2. List of presentation s and publications 31 2

3 List of abbreviations CT DMS-UM DT EFG ESR GDMS e-h HE ICP IFAC-CNR IP Lu x Y 1-x AP:Ce LY Computed Tomography Dept. of Material Science, University of Milano-Bicocca decay time Edge Film Growth Electron Spin Resonance Glow Discharge Mass Spectroscopy electron-hole High Energy Inductively Coupled Plasma Institute of Applied Physics N.Carrara (formerly IROE-CNR) Institute of Physics Lu x Y 1-x AlO 3 :Ce Light Yield Na67Gd3Ce3 an example of the glass composition meaning 67mol% of NaPO 3 - NC (PC) PG:Ce PET PL PMT PWO PbWO 4 RE RL RLU RT SME STE TSL UV VUV 3mol% ofgdpo 4-3mol% of CePO 4 in the melt NATO Country (Partner Country) Cerium-doped phosphate glass Positron Emission Tomography photoluminescence photomultiplier Rare Earth ions Radioluminescence Relative Luminescence efficiency Room Temperature small or medium enterprize self-trapped exaction thermally stimulated luminescence ultraviolet vacuum ultraviolet YAP YAlO 3 YAG (LuAG) Y 3 Al 5 O 12 (Lu 3 Al 5 O 12 ) 3

4 1. Introduction Czech Republic has undergone the change of political regime in 1989 and as a consequence the economical system was completely re-built as well, towards a market-oriented capitalistic economy. Due to political changes in most of the countries of Central and Eastern Europe, traditional markets changed or disappeared. At the beginning of nineties this caused very serious troubles to many enterprises in Czech Republic, which were massively exporting in this part of Europe, i.e. they were critically dependent on its economical situation. It was definitely not easy to quickly re-orientate the export towards more developed and more demanding markets in Western Europe, USA and other well-developed countries. Research, development and production of artificially grown single crystals has got a long tradition in Czech Republic. Industrial production of such materials was established already in late forties of last century and during the seventies and eighties it was well based in several companies. The forerunner of CRYTUR company (so called Monokrystaly Turnov ) was one of the major producers having around 7 employees at the end of eighties and with the production largely oriented in the military sector. Research of single crystal materials was spread among many research institutes and university laboratories and roofed under the Expert group of chemistry and physics of crystal growth based in 197 and converted into Czechoslovak Association for Crystal growth in 199, which is headed by K. Nitsch (member of IP Prague project team). After 1989, due to the breakdown of military concepts, the demand for single crystal production from this sector practically disappeared and the size of the company diminished considerably. After several ownership changes, the private company CRYTUR Ltd. was established in 1998 having about 4 employees and the production oriented on laser and scintillator single crystals based on doped aluminum perovskites and garnets using the original (Mo-crucible based) technology developed in the past. The research group of the Institute of Physics AS CR, Prague reached an extensive international experience and reputation in the field of scintillator materials research in the nineties. Especially the participation in the multinational Crystal Clear Collaboration coordinated from CERN and targeted collaboration with Italian and Japanese laboratories in the development of PbWO 4 scintillator gave the major impact. At the same time the domestic collaboration with the industry CRYTUR in Turnov, focused on the development of scintillators, was intensively running from about 1992 and in 1998 collaboration included the study of several material systems like CeF 3, PbWO 4 or Ce-doped aluminum perovskites and garnets. In this situation we have decided to apply for a NATO SfP grant in the field of scintillators, which offered the proper framework for our research&development intentions and for which we had very good material and research base in that time, supported by real joint research work with several laboratories abroad and the domestic industrial partner as well. The optimization of existing scintillation materials under production and the development of new ones was of crucial importance for such an industrial enterprise as CRYTUR, because of the need to establish and keep the production suitable for welldeveloped markets and to follow the latest trends in the scintillator research and their applications. The R&D activity of a small company could not solve such tasks alone and targeted national and international collaboration with research laboratories in the field was necessary for succesful development. 4

5 2. Scope and objectives of the project The aim of the project consisted in the preparation, characterisation, optimisation and reproducibility check of new/improved scintillation materials, which will significantly increase scintillation detector performances in several fields of application. Objectives in the original project plan: a) Preparation of Lu x Y 1-x AlO 3 :Ce 3+ single crystals, x=.3 and.8-1. for CT or PET applications, respectively. A 4+ optically inactive ion co-doping is proposed for LY maximisation. b) Preparation of Ce 3+, Tb 3+ and Pr 3+ -doped alkali metal-gd phosphate scintillation glasses. Optical features and scintillation performances. RL yield and slow decay components. c) Preparation of PbWO 4 single crystals doped by Mo 6+, Cd 2+ and S 2- ions. LY maximisation, slow decay component control. Co-doping by Y 3+ or Gd 3+ to keep high radiation hardness. Newly added objectives during the project: d) Preparation of Lu 3 Al 5 O 12 :Ce 3+ single crystals, role of defect states in the scintillation mechanism, A 4+ optically inactive ion co doping (Zr, Si). e) Mn and Yb-doped alkali metal-gd phosphate scintillation glasses. RL efficiency and radiation hardness characteristics. f) Doubly and multiply doped PbWO 4 single crystals like PWO:(Mo,X), X=Nb,Ta,V; triply doped (Mo,X,Zn); F- and Ba-doped and codoped PWO:Mo,F (Ba). RL efficiency, slow scintillation decay components. The content of the project was defined on the base of industrial interest of CRYTUR Ltd., trends in scintillator research in general, and available technological and characterization background in the cooperating laboratories. The backbone of the project was based on the existing collaboration between IP Prague group and CRYTUR Ltd., further completed with the existing domestic collaborations with Technical University in Prague and Liberec and mainly with well-developed international cooperations with laboratories in Italy and Estonia. Furthermore, collaboration with Japanese institutions was of advantage since it allowed to use in some cases their topquality materials for comparison and proper understanding of technological aspects. The training aspect was very important: due to limited research funds, during the nineties it was practically impossible to manage training of students or young researchers from Czech and Estonian teams abroad. The rules of SfP programme and the scope of this project offered very good opportunities for young people training abroad, which was in fact extensively used. Due to economical situation in Central and Eastern Europe the scientific infrastructure suffered from limited funds and innovation of the laboratory equipment was rather difficult. The amount of financial funds offered by SfP program for non-nato countries, completed in the case of Czech Republic by another domestic support for these grants from Ministry of Education, Youth and Sports and in the case of Estonia from Estonian Science Foundation, was of considerable help in this respect for laboratories in Czech Republic and Estonia. The project was based on collaboration of five research teams from three European countries and systematic collaboration with other external teams in Czech Republic, Italy, Ukraine, laboratories in CERN and in Japan. The performed studies, joint presentations and publications, mutual visits and project meetings created apart from professional interests also strong personal contacts, which will last also after project completion. 5

6 3. Realization of the project The management and project structure is given in Fig. 1 reflecting the coordination role of IP- Prague and straightforward interconnection with the industrial partner, which in fact had a double role of participating research laboratory and industrial end-user at the same time. Fig. 1: Organogramme of the project Institute of Physics IROE del CNR University, Bicocca University of Tartu Florence Milano Tartu Steady state & time-resolved polarisation spectroscopy down to.35 K TSL excitation spectra in UV- VUV energy range Zazubovich et al laser excited decay kinetics measurements emission spectra by OMA Pazzi et al wavelength-resolved TSL under X-ray or radioisotope excitations absorption and Raman spectroscopy Vedda et al Institute of Physics AS CR Prague technology of single crystal growth of doped PWO technology of doped phosphate glasses preparation time-resolved luminescence and radioluminescence, ESR Nikl et al CRYTUR Ltd. Turnov research of Lu x Y 1-x AP:Ce and LuAG:Ce single crystal growth industrial production, marketing and selling of developed scintillators Blazek et al NEW SCINTILLATOR MATERIALS Ce-doped (Lu-Y)AP and LuAG doped PWO doped phosphate glasses 6

7 Technological laboratories were included in CRYTUR and IP Prague teams, while characterization techniques were spread over all the project sites providing comprehensive technique set enabling application-related parameters evaluation and the study of basic physics phenomena, related to the energy transfer and storage, in the produced materials. External collaborations enabled an extension of characterization tools and in the case of PWO the comparison with topquality Japanese samples. 4. Scientific results In this chapter the results of major importance are reviewed with respect to every material group under study, related presentations and publications given in Annex 2 are mentioned. More details about the material preparation and characterization can be found in the Progress Reports, which contain a complete overlook of the performed activities. Three technological laboratories prepared a number of crystal and glass samples. In their characterization the application-related parameters were evaluated: total (radioluminescence) efficiency, light yield, scintillation decay and radiation damage. The aspects of basic physics were followed mainly by the timeresolved spectroscopy, TSL and ESR techniques which gave indications about the energy transfer and storage processes, defect situation and trapping levels within the forbidden gap. Defect occurence was correlated with the obtained practical performance namely as for the scintillation decay and radiation damage. 4.1 Ce-doped aluminium perovskites (Conference presentations: C-8, C-12, C-21, C- 22, C-3, C-31, C-32, C-33. Publications: 13, 16, 19, 22, 25, 31, 32, 4, 41, 44) Crystals of Ce-doped YAlO 3 (YAP) and Lu.3 Y.7 AlO 3 (LuYAP) were grown mostly by the Czochralski method and in the latter case also by EFG method. Purity of starting materials was always 5N for Al 2 O 3, 5N or 6N for Y 2 O 3 and 4N or 5N for Lu 2 O 3 oxides. Low concentration Ce-doped YAP (5 ppm of CeO 2 in the melt) of standard purity (both components of 5N purity ) was studied with the aim to investigate emissions of Ce 3+ centres at irregular sites (i.e. Ce 3+ residing at other-than-yttrium sites) and of the defects of the YAP host. In Fig. 2 the 337 nm (N 2 laser line) excited emission spectra are shown for both as grown and H-annealed samples. Decay times of the bands are reported in the figure, while the results of their decomposition into gaussian components are described in the caption. Besides the well-known Ce 3+ emission at 375 nm, two other fast bands at 5 nm and 68 nm were detected. By increasing the power density of the excitation on the sample, a reduction of the intensities of these two bands was observed. The decay time of the 5 nm band (17-19 ns), comparable to that of the 375 nm one (17-18 ns), gives the evidence of the presence of Ce 3+ ion at Al site, while the weak fast 2 ns emission at 68 nm might indicate the presence of Ce 3+ at an interstitial site. Annealing in hydrogen (H-annealing) was considered both for application purposes and for understanding the nature of the emission centres. In this intensity [arb.units] fast (17-19ns) blue-green fast Ce3+ regular (17-18ns) mediumslow (12 µs) red fast red (19.8 ns) y7599ag, ex=36nm,4.2k Y7765,H-anneal ex=337nm,rt Y7665-Han-em-3Gfit Y7665-ag-em-3Gfit Y7765,ag ex=337nm,rt wavelength [nm] slow red (.42 ms) Fig. 2 Emission spectra of as grown and H- annealed YAP 7765/8 sample at RT (λ exc =38 nm or 337nm). Gaussian decomposition of emission spectra gives spectral components at: 1) H-annealed: 1.88 ev, 2.49 ev and 3.24 ev. 2) as grown: 1.88 ev, 2.16 ev and 2.6 ev. 7

8 case, the peak of the emission band at 5 nm (excited by a N 2 -laser at 337 nm) is reduced and a little shifted at 48 nm while an intense band at 575 nm appears, with a RT decay time of about 12 µs. The 575 nm emission is different from the previously observed red emission (in Fig.2 at 6 nm,.42 ms decay time at RT) in several standard-concentration Ce-doped samples both for its position and decay time; moreover, it shows also a different response to H-annealing. ESR studies performed on this sample revealed several configurations of the (defect-stabilized) O - hole centre, which in some cases can be stable up to room temperature (Fig. 3). Such centres compete for the hole capture with Ce 3+ luminescence centres and are possibly related to the above described slow red emission of the host, while the 575 nm band could be related to an electron trap emission as noted earlier in the literature. Electron traps in this material seem to be connected with Y ions coupled to a defect in its nearest surrounding (oxygen vacancy), but ESR experiments did not yet provide a unique interpretation. O - centres are hardly detectable. In addition to the 37 nm emission, RL spectra occasionally show a weak emission in the red spectral region (identical with the slow red in Fig. 2) Fig. 4. Such emission is clearly undesired in a fast scintillator as it introduces another loss (even if radiative) channel. Stoichiometry tuning and postgrowth annealing can reduce this parasitic emission down to a negligible intensity. 1.2 intensity [arb.units] y7599 as grown yap7746 as grown y7599 H-annealed x wavelength [nm] Fig. 4. Radioluminescence spectrum of YAP:Ce at RT. Delayed radiative recombination and the presence of slower components in the scintillation decay, which shows a longer decay time with respect to the intensity [arb. units] I t I SS fast α=1%.i /I SS t decay components slow true background Fig. 3: ESR detected configurations of O- centres and their temperature stability in low concentration Ce-doped YAP [From C-31]. In standard concentration Ce-doped YAP (typically 5-1 ppm of CeO 2 in the melt) regular Ce 3+ centres and their emission at 37 nm prevail due to saturation of the above mentioned irregular and defect centres at much lower concentrations, so that also the time [ns] Fig. 5. Scintillation decay of YAP:Ce at RT, 511 kev photon excitation ( 22 Na radioisotope). The typical decay time of the fast component is 25-3 ns and that of the slow one is about 1 ns. The presence of very slow components in the tenshundreds microsecond time scale is monitored by the coefficient alpha sketched in the figure. 8

9 photoluminescence one (18 ns), is a well known phenomenon in YAP:Ce Fig. 5. In order to reduce the concentration of trapping sites (mostly electron traps), which are responsible for these phenomena, Zr-codoping was pursued. A set of YAP:Ce,Zr crystals with ZrO 2 concentration up to 1 ppm in the melt was prepared for these experiments. Optical measurements including transmission, PL and RL spectra and decay kinetics under the UV and 511 kev ( 22 Na radioisotope) excitations were performed. Positive influence of Zr codoping was found in speeding-up the scintillation response Fig. 6, both in the ns (evaluated by 1/e and 1/1 decay times) and tens-hundreds µs (evaluated by the coefficient alpha sketched in Fig. 5) time scales. Hydrogen annealing influence was tested and provided partial improvement on the scintillation response speed, too, especially for the crystals with the lowest ZrO 2 content. decay tim e [ns] decay time-1/e[ns] decay time-1/1[ns] H-ANNEALED YAP7746 alpha [%] ZrO2[mg/melt] 3 2 alpha [%] Fig.6. 1/e and 1/1 Ce 3+ emission decay times evaluated on YAP:Ce,Zr samples and their dependence on Zr concentration (4 mg is equivalent to about 1 ppm). On the right Y- axis the dependence of coefficient alpha is also reported. Influence of hydrogen annealing is shown for the highest Zr concentration. From Ref. 13. Zr-codoping was accomplished also for a set of four Czochralski grown mixed Lu. 3 Y. 7 AlO 3 :Ce crystals and a similar positive trend in the speeding-up of scintillation decay was obtained Fig. 7. A TSL investigation of Zr co-doped YAP:Ce crystals was performed, to put in evidence the role of such aliovalent dopant in the concentration of point defects giving rise to trap levels. The data shown in Fig. 8 display the effect of Zr-codoping on the TSL above RT. The different curves are labeled by the decay time [ns] DT1 - fast DT2 -slow alpha [%] mg of ZrO in melt alpha [%] Fig.7. Scintillation decay parameters of the set of Lu.3 Y.7 AP:Ce,Zr: dependence on ZrO 2 content. Two exponential approximation yields decay times DT1 and DT2; the coefficient alpha quantifies the relevance of very slow decay components. co-dopant concentration in terms of mg of ZrO 2 in melt. In these crystals the TSL glow curve appears complex, since several peaks are observed in the 2-4 C temperature region. It is interesting to remark that Zr concentrations of the order of 1 ppm and above are very effective in reducing the intensity of all glow peaks: this result is very positive and points to the possibility to strongly reduce the concentration of (possibly intrinsic) defects by a suitable codoping. In the case of the mixed Lu.3 Y.7 AlO 3 :Ce sample set the influence of Zr-codoping was possibly obscured by the lower quality of the host, because positive effect was obtained below 15 C, but new very deep trapping states contributed to TSL curves above 2 C, which might be related to structural imperfections of the material. The spectral distribution of the TSL features the expected Ce 3+ emission at around 3.4 ev. Moreover, another emission at 2 ev was 9

10 noticed, which spectrally coincides with the slow red emission displayed in Fig. 2. The intensity of such emission band does not seem to be related to the presence of Zr. A defect related origin can be assumed, possibly due to O - centers as mentioned above. TSL intenisty (arb. units) no Zr T [ C] Fig. 8. TSL glow curves of YAP:Ce,Zr crystals after x-irradiation at RT. Numbers in the figure give the concentration of ZrO 2 in mg/melt. From Ref. 19. TSL at low temperatures after irradiation at 1 K reported in Fig. 9 shows two dominant peaks at around 1 K and 15 K. While positive influence of Zr co-doping can be stated for the former one (the peak intensity was reduced), practically no influence was obtained for the latter, i.e. no change of the corresponding trap concentration occurred. For medium concentration of the Zr codopant (about 3-4 ppm in the melt) the concentration of Ce ions in YAP matrix was in several steps gradually increased up to almost 3 ppm (3%) to test scintillation performance of such heavily doped samples. Decrease of scintillation efficiency was obtained and no speeding-up of the scintillation decay was obtained, so that such material composition is not favorable for practical applications. The radiation hardness of the selected samples was studied by induced absorption measurements after irradiation by 6 Co radioisotope and by microtrone irradiation (irradiation doses from a few Gy up to about 1 Gy). The induced absorption spectra were decomposed into gaussian components tentatively ascribed to defects centres (example in Fig 1.). The most reasonable hypothesis about the origin of the induced absorption components is that G1, peaking at ev, and G2, at around ev can be related to a defect-stabilized O- centre and an electron trap, while G3 peaking at around 3.9 ev is related to Ce 4+ centre. TSL intensity [arb.units] no Zr 96 mg ZrO2 182 mg ZrO2 4 mg ZrO T [K] Fig. 9. TSL glow curves of YAP:Ce,Zr crystals after x-irradiation at 1 K. Numbers in the figure give the concentration of ZrO 2 in mg/melt. From Ref. 13. µ (cm -1 ) 1,8,6,4,2 Absorbance before irrad. after irrad. D=5 Gy 1,5 2 2,5 3 3,5 4 E (ev) G1 1,5 2 2,5 3 3,5 4 E (ev) Fig. 1. Radiation induced absorption coefficient µ= (Airr - A)/d evaluated from absorption spectra measured before (A ) and after (A irr,) irradiation with a 6 Co gamma source on YAP:Ce (dose = 5 Gy,. d stands for the sample thickness. The inset shows the absorption measurements before and after irradiation. Spectral decomposition in 3 gaussian components is also shown. From Ref. 19. G2 G3 1

11 The effect of the host material purity was tested too: for this purpose, 5N Y 2 O 3 was replaced by 6N powder from the same manufacturer. No substantial improvement was obtained neither in the scintillation efficiency, speed of scintillation decay nor in radiation resistance of the material. Systematic measurements of luminescence and trapping phenomena (temporary defect formation) were performed under VUV excitation in the region of YAP:Ce and Lu.3 Y.7 AP:Ce absorption edges. Temporary defect formation reveals itself in the appearance of the afterglow and TSL peaks whose intensity depends on E exc. At the afterglow excitation spectrum study, the crystal was irradiated for 1 min with the same number of photons of different energies E exc. Then the irradiation was stopped, and after 2 sec the afterglow intensity was detected. The intensity of the excitation light and the duration of the irradiation were the same for all the E exc values studied. Thus, the afterglow excitation spectrum is the dependence of the afterglow intensity on E exc. In both crystals, the luminescence is effectively excited not only in the Ce 3+ -related absorption bands but also in the exciton region (near 8 ev), while defects are more effectively created at around 7 ev than in the band-to-band transitions region (Fig.11). Under the 7 ev excitation, defects formation can be explained by photoionization of the 6s 2 S 1/2 excited state of Ce 3+ or by electron transfer from Ce 3+ -perturbed oxygen state. In both processes, Ce 4+ hole centres and electrons trapped round oxygen vacancies are produced. The recombination of the electrons with Ce 4+ centres results in the appearance of afterglow and TSL, whose spectra coincide with the emission spectrum of Ce 3+ centres. Co-doping with ZrO 2 leads to the decrease of defects creation efficiency and thus confirms its positive effect already mentioned for YAP:Ce and Lu.3 Y.7 AP:Ce scintillation characteristics. The quantum yield of Lu.3 Y.7 AP:Ce luminescence increases up to 3 ppm of ZrO 2 and then decreases. Luminescence quantum yield in Lu.3 Y.7 AP:1 ppm ZrO 2 crystal is smaller, and defects creation efficiency is much smaller than in YAP:Ce, 1 ppm ZrO 2 crystal. These dependences may be affected also by somewhat varying quality of the YAP and Lu.3 Y.7 AP hosts. Influence of Zr 4+ ions on the energy levels of Ce 3+ ions and on defect creation processes is much stronger in Lu.3 Y.7 AP:Ce than in YAP:Ce, probably due to a closer location of Ce 3+ and Zr 4+ ions in Lu.3 Y.7 AP:Ce. One may conclude that not only the excess charge of a co-dopant ion, but also the relation between ionic radii of the impurity ion, co-dopant ion and host lattice cation are important for the improvement of scintillation characteristics of these crystals (e.g., through the effective suppression of slow recombination processes). Intensity (a.u.) a b Photon energy (ev) Fig. 11. Excitation (curves 1, 2) and defects creation (curve 3) spectra measured in the same conditions at RT (curve 1) and at LNT (curves 2, 3) for Lu.3 Y.7 AP:Ce crystals without co-doping /no.839/ (a) and with 1 ppm of ZrO 2 /no.881/ (b). Open triangles, creation spectra of the TSL peak at K. From Ref. 44. Scintillation efficiency of the produced samples was instantaneously controlled by CRYTUR laboratories and compared to the company standard. Moreover, advanced light yield measurements (quantity of light emitted from the sample within typically 1 µs after high energy photon absorption) were performed using a hybrid photomultiplier

12 detector and a set of radioisotopes with energies from about 1 kev up to 511 kev. A typical measurement is shown in Fig. 12. Light yield values and energy resolution are given in Table I. including our own values obtained on reference scintillators to obtain a direct quantitative comparison of scintillation efficiency. no. of pulses 1 energy resolution = A/A 1 A A 15 channel number Fig. 12. Output curve from a multichannel analyzer in the measurement of light yield of YAP:Ce crystal under excitation of 137 Cs radioisotope (662 kev) at RT. The photopeak position (A ) provides information about the number of generated photoelectrons in the HPMT detector. The energy resolution is extracted by using the FWHM of the photopeak A shown in the figure. From Ref. P-5.. Crystal L.Y. [photons/mev] L.Y.* [photons/mev] En. Resol. [%] at 662 kev YAP:Ce ( 1 1 mm 3 ) YAP:Ce ( 1 2 mm 3 ) Lu.3 Y.7 AP:Ce ( 8 1 mm 3 ) up to 15 5 YAG:Ce ( 1 1 mm 3 ) up to LuAG:Ce ( 1 1 mm 3 ) LSO:Ce (2 2 7 mm 3 ) CsI(Tl) ( 1 1 mm 3 ) BGO ( 1 1 mm 3 ) Table I. Comparison of Light Yield (L.Y.) and Energy Resolution (En.Res.) of the Ce 3+ -doped perovskite and garnet crystals developed under this project compared to the reference LSO:Ce, CsI(Tl) and BGO scintillator crystals (scintillating plates of the same dimensions were measured). Values L.Y.* in the 3 rd column were cp;;ected from different references as e.g. {Kapusta et al. NIM Phys.Res.A 44 (1998), 413; Moszynski et al., NIM Phys. Res. A 44 (1998), 157; Lempicki et al., IEEE Trans.Nucl.Sci 42 (1995), 28; van Eijk C.W.E., Phys.Med.Biol. 47 (22), R85}. Measurements performed in collaborating laboratory of C. D Ambrosio in CERN

13 4.2 Ce-doped aluminium garnets (Conference presentations: C-8, C-17, C-28. Publications: 23, 24, 31,32, 38, 39, 4, 41, 43) Ce-doped aluminum garnet, namely Lu 3 Al 5 O 12 :Ce (LuAG:Ce), the heavier analog to YAG:Ce, was added in the project plan in 22 due to the rising perspective of this scintillator and interest of CRYTUR in its development. Contrary to LuAlO 3, the garnet Lu 3 Al 5 O 12 is stable and can be grown by Czochralski method similarly to YAG, the technology of which is well-developed in CRYTUR. In the first set of single crystals a lower 4N purity Lu 2 O 3 powder was used due to economical constraints (Lu 2 O 3 chemical is of a very high price) and an undoped and Cedoped crystals (up to.9% of Ce in the crystal) were prepared. In Fig. 13 VUV excited emission spectra of undoped LuAG are given showing bands peaking at around 25 nm (8 K) and 3-35 nm (RT). Intensity [arb. units] T=85K T=3K Intensity (arb. units) Wavelength [nm] nm 34 nm Temperature (K) Fig. 13. Luminescence of undoped LuAG under 7.7 ev excitation. In the inset temperature dependences of the two leading bands are shown. Emission, excitation spectra and their dependences on temperature and defects concentration were studied in detail for the two ultraviolet emission bands of undoped YAG and LuAG single crystals. Luminescence characteristics in both materials are very similar. Both these bands belong to the same centre and we can assume that they arise mainly from the exciton localized at antisite (Y Al 3+, Lu Al 3+ ) defects. The presence of these defects in the crystals can considerably quench the self-trapped exciton luminescence of YAG (~4.8 ev) and LuAG (~4.9 ev) observed at low temperatures in single crystalline films where antisite defects are absent (presentation of Yu. Zorenko, LUMDETR23 conference). In Ce 3+ -doped crystals the presence of these defects considerably decreases the efficiency of energy transfer to Ce 3+ ions. Systematic measurements were performed concerning decay kinetics of undoped YAG and LuAG in the temperature interval 4 3 K using pulsed excitation at the synchrotron ring (Desy, Hamburg), and in the 8 3 K range using ns pulsed X- ray excitation. The obtained decay curves were fitted by the sum of exponential functions and related decay times are given for the latter excitation in Fig. 14. Thermally stimulated transitions from the 25 nm to the 35 nm band are well-evidenced from the decay curve shape at around 15-2 K. decay time [ns] DT1-25 nm DT2-25 nm DT3-25 nm DT1-3 nm DT2-3 nm temperature [K] Fig. 14. Temperature dependence of decay times obtained from two- or three-exponential approximation of undoped LuAG decay excited by pulsed X-rays. Measurements performed in the cooperating laboratory of Prof. Voloshinovskii, I.F. university, Lviv. At room temperature decay time values in Fig. 14 support the idea of a two-level excited state of related emission centers consisting of radiative (fast) and metastable 13

14 (slow) levels mutually interconnected with multiphonon-assisted transitions. Radioluminescence spectra of undoped and Ce-doped LuAG samples are displayed in Fig. 15: a competition in energy capture between the centers responsible for UV emissions in Fig. 13 and Ce 3+ is evidenced. performance of LuAG:Ce scintillator and it is worth to look further for the diminishing of the (most probably antisite) defects responsible for exciton trapping in LuAG host. 5 Scintillation decay ( 22 Na) at RT LuAG 822 (.6%Ce) decay data 2exp. fit Intensity [arb.units] undoped.3% Ce.6% Ce BGO x1 intensity [arb.units] I(t)= 729exp[-t/53.8ns] + 27exp[-t/524ns] time [ns] Wavelength [nm] Fig. 15. Radioluminescence spectra of 4N LuAG samples at RT. We also remark the presence of sharp peaks at around 312 nm and 615 nm due to radiative transitions at the Gd 3+ and Eu 3+ trace impurities. Such contamination reflects the above mentioned lower purity of Lu 2 O 3 raw material. High integral scintillation efficiency of LuAG:Ce is demonstrated by the fact that.6%ce sample RL shows practically ten times higher RL intensity with respect to BGO. Scintillation decay of Ce-doped LuAG shows an initial decay constant of about 5-6 ns, in accordance with the photoluminescence decay time (about 54 ns) Fig. 16. With respect to perovskites slow components are more intense, especially those very slow ones characterized by the coefficient alpha Table II. Due to the overlap of the 3-35 nm emission of the undoped LuAG at RT with the absorption bands of Ce 3+ and similarity of the decay times of slower component of the former (Fig. 14) and of the scintillation decay (τ 2 in Table II.) the energy transfer from trapped exciton to the Ce3+ center can be concluded. This phenomenon degrade timing Fig. 16. Scintillation decay of 4N LuAG:Ce.6% at RT. Table II. Parameters of 2-exponential fit I(t)=ΣA i exp[-t/τ i ] +constant, of the scintillation decay of the 4N LuAG:Ce sample set Ce con. crystal A 1 τ 1, [ns] A 2 τ 2, [ns] α, [%] DT 1/e [ns].3% % % % TSL after x-ray irradiation at RT is shown in Fig. 17 and features several intense peaks. While some of the peaks decrease by increasing Ce concentration, others display none or even opposite dependence, so that deep trapping states of different origin should be present. The lower TSL intensity of the undoped sample is most probably due to the lower quantum efficiency of recombination centres with respect to Ce 3+, the principal recombination centre in the doped samples. Following a similar strategy as that used in the optimization of Ce-doped perovskites, Zr-codoping was pursued as well in 4N LuAG:Ce samples, but the 14

15 obtained results were rather different. Namely, no speeding-up of scintillation decay was achieved and TSL above RT showed a drastic enhancement Fig. 18. This probably means that the presence of Zr gives rise to other (electron) traps (existence of Zr 3+ was evidenced earlier in YAG in the literature by ESR experiments). Such a difference between perovskite and garnet structures may be due to the very different crystal field strength at Y site: the strong field in the garnet might stabilize Zr 3+ level within the forbidden gap. TSL intensity (arb. units) undoped.3% Ce.4% Ce.6% Ce Temperature ( C) Fig. 17. TSL glow curves after x-irradiation at RT of 4N LuAG:Ce samples. In the next optimization step, 5N Lu 2 O 3 raw material was used and another set of LuAG:Ce crystals was prepared up to.14% Ce concentration in the crystal. A significant improvement of the alpha value was achieved (α=5.6% for.12% Ce), RLU and LY was increased by about 1-15% and TSL intensity above RT was decreased by more than one order of magnitude (comparing the leading 25 C peak in equivalent 4N and 5N LuAG samples). Nevertheless, similarly to 4N LuAG:Ce, TSL at low temperatures shows a complicated behaviour Fig.19: by increasing Ce concentration the maxima round 15 K are decreasing. But the opposite trend is obtained at heigher temperatures. Such characteristics point again to the complex character of trapping phenomena in these materials. TSL intensity LuAg:Ce, Zr 4 ppm Zr 19 ppm Zr 5 ppm Zr x 1 No Zr Temperature ( C) Fig. 18. TSL glow curves after x-irradiation at RT of Zr-codoped 4N LuAG:Ce samples.from Ref.24. TSL intensity (arb. units) % Ce.7% Ce.12% Ce Temperature (K) Fig. 19. TSL glow curves after X-ray irradiation at 1 K performed on 5N LuAG:Ce samples. Under VUV excitation the same experiments were performed as shown in Fig. 11 for Cedoped perovskites. Very similar excitation spectra of photoluminescence and defect creation (afterglow) were obtained, just shifted to lower energies by about 1 ev reflecting the narrower forbidden gap in garnets with respect to perovskites. Such similarity points to analogous positioning of Ce 3+ energy levels within the forbidden gap in both materials. Several announcements were reported in the literature about possible quenching (loss) effect due to closely spaced excitations created during the picosecond 15

16 scintillator conversion in the stage of final interactions of hot electrons (of the energy of few hundreds of ev) with the dense lattices (e.g. Nikl et al, J.Phys.Cond.Matter 7, 6355(1995)). An attempt was made to prepare 5N LuAG:Ce crystals with a small admixture of Y 2 O 3 (5% and 1%), which should limit such phenomena. However, no substantial increase of the integral efficiency (radioluminescence intensity) was obtained and similarly to the 4N sample set the best crystals have shown about 1 time higher RL intensity with respect to BGO in comparable conditions. Light yield measurements were performed for selected samples and in the best 5N purity samples about 6% of YAG:Ce and about 15% of BGO was obtained (see Table I.). At the same time, the integral scintillation efficiency (radioluminescence intensity) mentioned above was up to 1% of BGO. This points to the considerable presence of re-trapping processes and delayed recombination at Ce 3+ centres, due to the existence of shallow traps in the material. In further search for co-dopant ions suitable for oxygen vacancy (and derived electron traps) reduction, Si 4+ co-doping was pursued only for YAG crystals: about 1 ppm and 1 ppm of SiO 2 was added in the melt during the crystal growth. In the former case crystal quality remained very good and the light yield was increased by about 1% with respect to Si-free YAG:Ce crystals with similar Ce concentration. In the latter case, crystal quality seriously degraded and parasitic absorption bands appeared in the transparency region, causing scintillating light lowering due to reabsorption. However, by increasing Si concentration, an encouraging result was obtained: namely, the radiation resistance was increased especially for the higher Si doping level Fig. 2 Indu ced absorption coefficient (m -1 ) 8 4 undoped.12% Ce.32% Ce.25% Ce. low Si.25% Ce, high Si 1 1 Dose (Gy 1 Fig. 2. Radiation induced absorption for 6N YAG:Ce;Si sample set. Decreasing value of the induced absorption (i.e. increasing radiation resistance) with higher Si concentration is well observed for irradiation dose above approx. 5 Gy (irradiation by 6 Co radioisotope). Measurements performed in collaborating laboratory of Dr. Baccaro Doped PbWO 4 (Presentations C-1, C-3, C-6, C-1, C-11,C- 14, C-15, C-16, C-19, C-2, C-23, C-24, C-25, C-26, C-27, C-34. Publications 2,3,4,5,7,1,11,14,15,18,2 21, 27,28,29,3,33,34,35,36,37,42) Crystals were grown by Czochralski method using 5N purity raw powders of PbO and WO 3. Single doping by Cd or S did not give satisfactory results (no or small light yield enhancement), so that our attention was focused on doubly or multiply doped systems and later on the attention was paid to the nonstoichiometric melt-based crystals with eventual single or double doping. A set of crystals doped by Mo and (Mo,Y) in a wide range of Mo concentration (2 5 ppm) was prepared and studied. In addition to the intrinsic PWO emission peaking at 42 nm, Mo doping introduces another emission centre with the emission band peaking at around 5 nm (green 16

17 emission component), the intensity of which depends on the Mo concentration as it can be seen from the radioluminescence spectra Fig. 21. Maximum RL intensity is obtained for Mo concentration between 3-1 ppm; for larger concentrations the self-organization of PbMoO 4 phase occurs, in which the emission is severely quenched at RT. Co-doping by Y at the 1 ppm level resulted in a partial decrease of the total efficiency (RL intensity). However, a considerable speed-up and decrease of slow decay processes introduced by Mo doping and observed both in PL and scintillation decays occurred Fig. 22 and 23, respectively. Intensity [arb.units] Wavelength [nm] undoped 2 ppm 1 ppm 275 ppm 1 ppm 5 ppm Fig. 21. RT radioluminescence spectra of Modoped PWO. Concentration of Mo in the melt is given in the legend. From Ref. 1. intensity (a.u.) Mo16 Mo16Ce4 Mo16Y8 Mo275Nb time (s) Fig. 22. PL decay of Y-codoped PWO:Mo crystals at 5 nm and under excitation by an excimer laser (XeCl line at 38 nm) at RT. Time domain of strongly different slow decay processes in the samples is marked. From ref. 11. sity [arb.units] Inten 1 275/ppm I T I S 275/1ppm α=(i S / I T )x1% true background As a result, in PWO:Mo,Y (1 ppm, 1 ppm) the light yield was increased by a factor of two and a scintillation speed comparable to that of the undoped crystal was maintained. Such material can enhance the performance of scintillation detectors in high energy physics experiments like ALICE in LHC. Time [ns] Fig. 23. Scintillation decay of PWO:Mo and PWO:Mo,Y crystals, doping concentrations given in the legend. The mean decay times of 22.3 ns and 1.5 ns and alpha coefficient of 1.1% and.55% were calculated for the Y-free (275/) and Y-doped (275/1) samples, respectively. From Ref

18 TSL intensity [arb.units] undoped (x.5) 2/ ppm 2/1 ppm Temperature [K] Fig. 24. TSL glow curves. of PWO(Mo,Y) after X-ray irradiation at 1 K. Comparison between Y-free and Y-codoped samples is shown; concentrations of Mo and Y dopants are given in the legend. From Ref. 1. Furthermore, a positive effect of Y co-doping was discovered also for what concerns the radiation resistance of such PWO:Mo,Y crystals. Decrease of concentration of trapping states introduced by Mo-doping was demostrated by TSL measurements, especially for those giving rise to peaks in the 2-25 K interval Fig. 24. In order to increase further the scintillation efficiency of PWO-based scintillator, codoping of PWO:Mo by pentavalent ions was undertaken. In this case, more than 2 times increase of RL intensity was achieved for the optimized Nb 5+ concentration and the RL intensity became comparable to that of BGO Fig. 25. However, the simultaneous presence of Mo and Nb ions in the PWO lattice gave rise to a new trapping level related to a TSL peak at around 1 K Fig. 26: it resulted in a noticeable increase of slow decay components, (see Fig. 22) and is reflected also in the coefficient alpha inset of Fig. 26. Partial improvement was achieved by codoping the third ion, namely Y or Zn. In the case of the optimized Nb-codoped samples (see the dependence of RL intensity on Nb concentraiton in the inset of Fig.25), a light yield value up to three times of that of an undoped PWO was obtained. RL intensity [arb.units] 2 1 PW O(264,31,5) PW O (276,35,) BGO PWO(226,,) x4 PWO und.x2 PWO(,35,) x Nb concentration [ppm] wavelength [nm] Fig. 25. Radioluminescence spectra of PbWO 4 (Mo,Nb,Y) and BGO samples at RT. True dopant concentrations in the samples are indicated in parentheses. In the inset normalised RL intensity is given as a function of Nb concentration: the measurements were performed on sample plates cut in different positions along the PWO(275,5,) parent crystal. From ref. 11. The results obtained in the co-doping by triand pentavalent ions point to a possibly important role of oxygen vacancies in radiative recombination and light production processes in PWO scintillator. It is also known that, apart from Mo-doping, the oxygen vacancy itself introduces another green emission centre, which is usually recognized as WO 3 group. That is why we have tempted the growth of PWO codoped by monovalent anion (fluorine) and mainly the growth from the non-stoichiometric (PbO deficient) melt, which can enhance the creation of such emission centres. While the F-codoping did not show any noticeable RL intensity increase, the latter approach appeared very effective for this purpose. The most encouraging result is shown in Fig. 27, in which Mo-doped RL intensity [arb.units]

19 PWO grown from non-stoichiometric melt displays a RL intensity even exceeding that of BGO. Enhanced green emission was obtained also in Ba-doped crystals, in which this effect may be induced by the Ba-driven crystal non-stoichiometry and local stress combined effects. TSL intensity (arb. units) alpha [%] PWO:Mo,Nb PWO:Nb PWO:Mo undoped Temperature (K) Fig. 26. TSL glow curves of different PWO crystals (vertically shifted for better visualization) after X irradiation at 1 K. Heating rate =.1 K/s. The glow curves were obtained after integration of wavelength resolved measurements in the nm range. The inset displays the TSL integrals in the 8-31 K interval versus coefficient alpha. From Ref. 11. Such huge emission intensity changes after co-doping or due to stoichiometry tuning are rarely achieved in a material. In PWO, the underlying reason is related to the thermal instability of the emission centre excited states (both of the self-trapped exciton emitting at 42 nm, and of the green emission centres), which become thermally disintegrated into free electron hole pairs at sufficiently high temperatures. Such phenomenon is well-known in semiconductors, but not in wide band-gap covalent-ionic materials. In other words, luminescence quenching, a well-known phenomenon in PWO, is not driven by intracenter nonradiative transitions, but rather by band-to-band nonradiative TSL integ.(arb.un.) Intensity [arb. units] PWO:Mo ns BGO Wavelength [nm] Fig. 27. Radioluminescence of nonstochiometric Mo-doped (6 ppm) PWO at RT compared to that of a standard BGO sample recombination of electrons and holes created from the above mentioned thermal disintegration of emission centres Fig. 28. In such a situation any new emission centre, which is able to capture these migrating electrons and holes, and to induce their radiative recombination, can noticeably change the proportions between the overall radiative and nonradiative deexcitation processes. Fig. 28. Overall sketch of the excited state dynamics of the STE and (MoO 4 ) 2- centers. Ionisation energies of the excited states of the centers are shown and non-radiative electronhole recombination is recognized as the principal nonradiative quenching process in PWO. From Ref

20 On the contrary, introducing the efficient nonradiative trap will result in faster PL and scintillation decays with severely suppressed slow recombination processes and lower RL intensity: this effect was clearly demonstrated by the Ce-codoping of PWO:Mo Fig. 22. The evidence of such excited state ionization processes was obtained by TSL measurements after UV irradiation at the wavelength tuned to the excitation band of the STE or (MoO 4 ) 2- centres. TSL glow curve amplitude is then dependent on the irradiation temperature - Fig. 29. From the temperature dependence of TSL glow curve integrals the activation (ionization) energy can be calculated. The obtained ionization energies govern the temperature dependences of the decay times of the blue and green components especially at temperatures above 2 K. As an example, the temperature dependence of the 5 nm emission decay time and its phenomenological model are given in Fig. 3. Intensity [arb.units] Temperature [K] 18 K 17 K 16 K 15 K 14 K 13 K Fig. 29. TSL glow curves obtained at λ em = 5-52 nm after 15 min irradiation at 33 nm (FWHM = 6 nm). Irradiation temperature is given in the legend. From Ref. 27. decay time (ns) k ED e-h pairs k 21 k 1 D k 2 k 12 level 2 level 1 k ED e-h pairs Temperature (K) Fig. 3. Temperature dependence of the decay times of the green emission component based on the (MoO 4 ) 2- centers, exc=33 nm, em=52 nm. Experimental data are given by circles. Solid line is calculated from the two-excitedstate level model shown in the inset and parameters are given in Table III. From Ref. 27. Table III. Parameter values used in in the calculation by applying the model showed in the inset of Fig. 3. Parameter K is a zero temperature rate related to k12 and k21 nonradiative processes. The rate of the ionization process is considered in a standard form k ED =w exp(-e ED /kt), where w ED, E ED and k stand for frequency factor, activation energy, and Boltzmann constant, respectively. k s -1 w ED s -1 k s -1 E ED.26 ev K s -1 D.35 mev A systematic study of electron traps in PWO was performed by parallel ESR and TSL studies up to room temperature. The complete scheme with the kind of centres and position of their levels within the gap is sketched in Fig. 31. From these results the crucial role of oxygen vacancy in deep electron trap creation is evidenced. At the same time, these results allow to conclude that electron traps responsible for stable radiation damage phenomena (i.e. 2

21 stable at least for several tens of minutes at RT) must be complex, i.e. consisting of an oxygen vacancy coupled to another defect(s) nearby. of traps due to irregular glass microstructure resulting in extended radiation damage, we attempted to realize a so called energy guiding sublattice, which would allow more efficient energy delivery towards emission centres and limited interaction of absorbed energy with the glass matrix defects and traps. The glasses were prepared by pouring of homogenized melt of sodium Gd and activator phosphates into preheated graphite crucible and postpreparation annealing was applied afterwards to remove considrable stresses in the material monitored in polarized light. The concept of emergy quiding sublattice in the glass matrix was realized first for the Ce-doped Gd-rich phosphate glasses, in which energy is captured in the Gd-sublattice and after some migration over Gd 3+ ions it is transferred towards Ce 3+ ions. A RL intensity enhancement was found of the order of several hundreds percent with respect to the Gd-free glasses with the same Ce concentration as can be seen in Fig. 32. Fig. 31. Electron traps in PWO: their nature and position within the band gap (in the upper part) and their temperature stability with accompanying TSL peaks arising during their thermal disintegration (at the bottom). From Ref RE-doped sodium-gadolinium phosphate glasses (Conference presentations C-2, C-4, C-5, C-7, C-9, C-13, C-18, C-29; Publications 1,6, 8, 9, 12, 17, 26, 45) Scintillating glasses are used in less demanding applications mainly due to their lower production cost, easy shaping and versatile composition. To overcome their main drawbacks lower efficiency resulting in low light yield, and a number intensity [arb.units] 2 x x1 4 x.16 1 x1 4 5 x1 3 emission of Gd 3+ Ce 3+ concentration Na/Gd/Ce mol.% 99//1 97//3 74/25/1 67/3/3 65/3/5 7/3/ x wavelength [nm] Fig. 32. Radioluminescence spectra of phosphate glasses at RT (X-ray excitation, 35 kv, Mo anticathode). Concentrations of the Na, Gd and Ce ions are given in the legend. The spectra are mutually comparable in an absolute way. From Ref

22 A similar effect was obtained for Tb 3+ and Mn 2+ doped Gd 3+ -rich glasses. Efficient energy transfer from Gd 3+ ions towards Ce 3+, Tb 3+ or Mn 2+ luminescence activators can be evidenced also from the shortening of the Gd 3+ (donor) decay curve Fig. 33. From the course of these decay curves, more details about the energy transfer interaction mechanism can be extracted. Contrary to the situation obtained in crystalline systems where such Gd-Gd...- Gd-Ce(Tb, Mn) energy transfer processes are very efficient, in phosphate glasses the migration over the Gd sublattice appeared more limited. The temperature dependence of the Gd 3+ and Tb 3+ transitions intensities provided a further evidence about the limited resonance conditions between neighbouring Gd ions, due to crystal field fluctuations in the glass matrix. a material. In the case of Ce 3+ doping another drawback appeared: even reverse transfer Ce 3+ - Gd 3+ was noticed at higher temperatures, which introduces the slow decay components in Ce 3+ luminescence Fig. 35. Stokes shift Intensity (arb. units) ,12,1,8,6, a b 3,994 3,992 3,99 3,988 3,986 3,984 3,982 3,98 ev intensity [arb.units] b a decay time 6.5 ms mean decay time.52 ms time [ms],2, Temperature (K) Fig. 34. Temperature dependences of the peak position of the Gd 3+ emission band (curve 1), and maximum intensities of the Gd 3+ (curve 2) and Tb 3+ (curve 3) emissions measured for the Na62Gd28Tb1 glass under excitation into the Gd 3+ -related 4.52 ev absorption band. (b) Temperature dependence of the Stokes shift of the Gd 3+ emission (the same in undoped and in all the doped Gd-enriched glasses). Fig. 33. Photoluminescence decay time of Gd 3+ centres (exc.=275 nm, em.=312 nm) at RT for Na7Gd3Ce sample (curve a) and Na74Gd25Ce1 sample (curve b). Namely, below 1 K the energy migration is practically frozen. With temperature increase it slowly appears which results in gradual decrease of the Gd 3+ emission, and accompanied also by its Stokes shift decrease Fig. 34. Such diffusion limited energy transfer results in a rather low scintillation efficiency of such Even if at the wavelength of the Gd 3+ lowest energy absorption transition (312 nm) there are virtually no absorption states belonging to the phosphate glass matrix, residual low density states in the band tails are most probably responsible for interaction with the excited Gd 3+ ions and limited energy transfer to the glass defects does occur. Evidence about such processes was obtained by monitoring Tb 3+ emission intensity at 542 nm over long times under the intense XeCl (38 nm) excimer laser irradiation. In some cases a noticeable 22

23 emission intensity increase was obtained at RT Fig. 36, which can be explained as a progressive filling-of-traps and more efficient energy transfer towards the activator ions. intensity (a.u.) SC content [%] K 2 K slow comp. increase I = (A SC 2 τ + A )/(A 2 3 τ + A 3 1 τ + A ) 1 2 τ 2 3 τ 3 35 nm 325 nm T [K] 3 K time [ns] laser 19 K 2 K 3 K fit 3K Fig. 35. PL decay of Na52Gd45Ce3 phosphate glass at different temperatures; in the inset the percentage of the slow components I SC vs. T is given. I SC values were obtained using a 3- exponential approximation of the decay curve (I(t)=Σ A i exp[-t/τ i ], i=1,2,3) ; the equation is given in the inset. The fit given by solid line at 3 K decay curve gives the values: A 1 /τ 1 =1/19 ns, A 2 /τ 2 =.11/123 ns, A 3 /τ 3 =.58/393 ns. Systematic attention was paid to the radiation resistance of Ce-doped and Tbdoped glasses. In the case of Tb-doped ones a deep analysis of the induced absorption spectra after X-ray irradiation was made, which revealed the creation of Tb 4+ absorbing at 3.4 ev and other color centres related to the phosphate groups Fig. 37 and Table IV. Yb 3+ -doped glasses were also prepared to examine a possibility to obtain an efficient charge transfer luminescence of Yb 3+ center and radiation damage mechanism as well (the opposite trapping ability with respect to Ce 3+ and Tb 3+ can provide a deeper insight into trapping mechanims in these materials). However, no efficient PL or RL was obtained at room temperature. The induced Emission intensity (arb.units) a b Time (min) Fig. 36. Dependence of the Tb nm emission line intensity upon irradiation time (UV excitation 38 nm, XeCl excimer laser line, pulse power of 13 kw/mm2) at 295K (curve a) and 25 K (curve b) for the Na77Gd2Tb3 glass. From Ref. 6. Abs. coeff. (1/cm) Abs. coeff. (1/cm) Tb3Gd as received Energy (ev) Tb3Gd x-irradiated Energy (ev) Fig. 37: RT optical absorption spectrum of as received and x-ray irradiated Na97GdTb3 glass.experimental data by symbols;solid line, numerical fit; short dashed line, spectral components. From Ref

24 absorption spectra confirmed the ascriptions made in the case of Tb-doped phosphate glass mentioned above. From the application point of view, the limited concentration of Gd 3+, which could be achieved in the produced glasses (up to 45% of GdPO 4 in the melt) and limited energy migration over the Gdsublattice did not allow to reach more than about 15% of CsI:Tl standard scintillator radioluminescence intensity for any kind of activator (Ce,Tb,Mn) luminescence centres. Table IV. Peak energies and full widths at half maximum (FWHM) of the gaussian components obtained by the numerical fit of the optical absorption spectra of as received and x-irradiated glasses. The measurements were performed at RT. E (FWHM) E (FWHM) E (FWHM) E (FWHM) E (FWHM) E (FWHM) E (FWHM) E (FWHM) unirradiated Na97GdTb3 x-irradiated unirradiated Na75Gd25Tb x-irradiated [ev] [ev] 2.3(.2) 2.9(.5) [ev] 3.4(.8) [ev] 4.(1.3) [ev] 4.2(.5) [ev] 4.9(.2) 4.9(.2) 4.9(.5) 4.9(.6) [ev] 5.(.7) 5.1(.8) [ev] 5.8(.3) 5.9(.3) 4.5 Impact of the project on the present and future research activities in the participating teams In the team of IP Prague the research direction on scintillation materials was distinguished as a strategic direction within the institute future research plans and it is involved in the research conception of the institute until 21 year. In the case of phosphate glasses, another domestic Czech grant was awarded (K. Nitsch) for the period focused on the preparation technology and properties of glass matrix. Such research will very probably bring increased efficiency of scintillation with respect to the parameters achieved so far. Encouraging results achieved in the case of nonstoichiometric PWO will be the base for another domestic project application, which is in preparation for April 24 deadline (P. Bohacek) and will be focused on high light yield limits of PWO-based scintillator. Comprehensive research accomplished on Ce-doped Al perovskites and garnets distinguished possible key defects in these structures, which limit scintillation performance especially in Lu-containing systems: antisite defects are considered and extensive search will be made to understand more deeply their role in the energy transfer and storage processes in these materials and hopefully diminish their concentrations. Focusing on this matter, we have already applied (M. Nikl) for an INTAS grant within 6 FP EU program in autumn 23 (some members of this consortium together with another laboratories from Ukraine and Russia), the project was well rated, but unfortunately not funded due to limited funds available and many applications received. We are considering a possibility to apply with this topic for another domestic project in Czech Republic and for NATO Collaborative Linkage grant as such research can fit the present priority areas defined in NATO scientific policy. 24

25 The research activity on scintillating crystals and glasses became very well based also in DMS-Milano, supported by an ongoing domestic project of the Ministry of Education and Research (23-24 years). Two master theses were recently accomplished on the closely related arguments, and another master thesis is being performed. Recently, one PhD student has joined the group whose research task during next three years will be related to the optical properties of scintillators. We are considering a domestic project application on scintillating glasses for the next year, whose application deadline should be within April-May 24. In the Institute of Physics, University of Tartu, the study of various scintillation materials will be continued in the framework of the Estonian Science Foundation projects (the present one will be terminated in the end of 24, the new project will be submitted for the period 25-27) and planned international projects in close collaboration with the partners from Czech Republic, Italy, Ukraine and Russia. We plan to study undoped and doped single crystals and thin crystalline films of yttrium-aluminium and lutetium-aluminium perovskites and garnets and to continue the study of undoped and doped lead tungstate crystals. The luminescence of these materials will be investigated by the steady-state and time-resolved polarization spectroscopy methods in the temperature range.35-5 K. Photo-thermally stimulated defects creation processes will be studied by thermally stimulated luminescence method in the K temperature range after irradiation of the crystal in the UV-VUV spectral region ( ev). For the future, the team of Florence is interested to continue the investigation of the optical properties of scintillating materials (PWO, perovskites, garnets, heavy-metal oxide glasses) by performing decay-kinetics and time-resolved optical spectroscopy measurements. As for the financial support the team will utilize the domestic funds. It is intended to continue the strong cooperation gained among the laboratories involved in the three years of NATO SfP Project for the investigations of common interest. Their complementary activities will be very fruitful. In the framework of the cooperation with the IP Prague, a Joint Project for years was presented to CNR and Czech Academy of Sciences with the title New scintillation and laser solid state materials, which is under evaluation. Even the cooperations with the Dept. of Material Science of the University of Milano Bicocca, the factory CRYTUR and the IP of Tartu will continue. Nowadays Dr. S. Zazubovich of IP of Tartu is working in Florence laboratory as the winner of a six-months NATO-CNR Fellowship for senior scientists. Furthermore, the cooperation with the laboratory of ENEA-Casaccia is going on for the investigation of heavy-metal oxide glasses. 25

26 5. Meeting the criteria for success and Implementation of the results The amount of performed research work and related results considerably exceeded the original project plan, because new material Lu 3 Al 5 O 12 :Ce was included in 22, developed, characterized and optimized in several aspects. Furthermore, other, not planned doping combinations were realized both in PWO and phosphate glass materials with the aim to obtain more comprehensive knowledge about physical properties and application potential of these systems. In terms of deliverables and statements in the Success Table the situation is as follows, percentage stated in the Success Table is given, if applicable: 1) In all the materials under study the luminescence and scintillation mechanisms were described, understood and results were or are being published, optimum doping concentrations were determined in all the cases of practical importance (3%). 2) We met deliverables as for the size of the developed crystals or glasses Fig. 38, 39 and 4. In the case of Zr-codoped YAP even large size crystal was already succesfully prepared in the industrial production of CRYTUR - Fig ) We have developed doped PWO with the radioluminescence output comparable with BGO (even two compositions are available Fig. 25, 27) and in the case of interest CRYTUR can supply to anyone the segments up to 1x1x5 mm in collaboration with IP Prague technological laboratory (1%). This result was patented in Czech Republic (M. Nikl, P.Boháček: Scintilátor na bázi wolframanu olovnatého. PV (July 21). 4) We developed Lu.3 Y.7 AP:Ce with light yield over 17% of BGO and Zrcodoped Lu.3 Y.7 AP:Ce and YAP:Ce which are at least 2% faster with respect to standard YAP:Ce produced in CRYTUR before (1%). 5) We failed to develop single crystals of Lu.8 Y.2 AP:Ce. However, considerable knowledge was acquired about the bottlenecks in this technology, which will be useful in further developments. 6) We developed new and important scintillation material Lu 3 Al 5 O 12 :Ce, with the light yield over 15% of BGO, which is commercially available from CRYTUR in the dimensions at least 2x3 mm, Fig. 4 (2%). 7) We did not reach quantitative performance in terms of radioluminescence intensity in the case of phosphate glasses. Present samples are commercially available to anyone up to the dimensions 1x1x5 mm from CRYTUR in collaboration with IP Prague technological laboratory. 26

27 Fig. 38. Czochralski grown PWO single crystals of 12 mm diameter and length of about 5mm and 4 mm. Fig. 39. Ingots of Ce-doped Na-Gd phosphate glasses. From the left: (i) as prepared ingot of about 12x12x6 mm; (ii) cut and polished segment 1x1x5 mm; (iii) not polished glass cut. 27

28 Fig. 4. Cut of Lu 3 Al 5 O 12 :Ce of 2 mm diameter and 3 mm in length with polished front and back faces. Fig. 41. Zr-codoped YAP:Ce of 45 mm diameter and 125 mm in length with cut and polished front face. 28